Not to be confused with FTP tunnelled over an SSH connection (which is made complicated by the fact that FTP requires multiple connections, the endpoints of most of which are not generally known at the time the tunnel is created).
Client for Windows available [here] (yes, it's PuTTY). OpenSSH? includes a client, so that's probably the way forward for *nix or Cygwin. For those of you who like your GUIs, WinSCP also supports SFTP.